1. Field of Invention
The invention relates to thermal insulation with a double-walled housing with an evacuated space between the two housing walls in which at least one layer is arranged perpendicularly to the temperature gradient and impedes the heat flow between the two housing walls.
2. Description of the Prior Art
Such thermal insulation is used in power technology, especially, in apparatus in which heat losses are to be avoided. Thermal insulation is used, among other things, in high-temperature storage batteries of the alkali metal and chalcogen type. The storage cells are surrounded by thermal insulation in order to prevent them from cooling off, particularly during the operating pauses of the high-temperature storage batteries, so that after the pauses a temperature of 350.degree. C. prevails in the storage cells, which is a necessary condition for optimum operation of the storage cells.
German Published Non-Prosecuted Application DE-OS 28 19 026 discloses an electrochemical storage battery with thermal insulation. The insulation is formed by an evacuated space, within which metal foils are arranged. The metal foils are arranged perpendicularly to the temperature gradient. The spacing of the metal foils is very small.
A disadvantage of this insulation is that it cannot be stressed in compression without a steep increase in its thermal conductivity. If it is loaded by a weight, the metal foils are pressed together whereby large-area contact points between the individual foils are increasingly formed. Thereby, the solid share of the total thermal conductivity of the thermal insulation is increased to values which can no longer be neglected.
Also known is a thermal insulation for high-temperature storage batteries which is composed of glass or mineral wool. In this insulation, considerable wall thicknesses must be provided to obtain an effect sufficient to keep the high-temperature storage batteries from cooling off too fast especially during the operating pauses, and also to maintain the operating temperature of 350.degree. C. for a long time. Since such thick-walled insulation increases the dimensions and/or the weight of the high-temperature storage battery considerably, the energy storage density, i.e., the energy that can be stored per unit of weight or volume is small.